She is an elf from Lorien.
None in our table are Tolkien hardcore fans nor anything like that, the book says spiders is a valid enemy lore and I imagine she disliked spiders enough in real life so she chose that.
As I'm not constrained by Tolkien lore I didn't mind introducing spiders to the west, also she may have her reasons to know about them, maybe an old tutor was from Mirkwood, maybe she traveled to Mirkwood and learned about the shadow there and returned to make a difference.
What I'm planing is to make this particular nameless spider thing very discrete, basically treading a web of darkness and corruption in Eriador like the mafia or a drug cartel, corrupting people in all places and staying hidden, when the campaign is over the spider menace in the west may be over and people will probably know nothing about it, because it will be like fighting a shadow war against the mafia.
That is the campaign concept, many smaller enemies and conspiracies like a spider web, linking some merchant from the Dalelands to some official in Bree, and then with some road muggers and hidden orcs in the hills, and so on, at the center of it all is the nameless thing pulling the strings. Each adventure will take them closer to the web center. Or at least that is the general plan...